Long-Term Seagrass Monitoring in Port Curtis: quarterly permanent transect monitoring progress report 2009 to 2015

Key findings: This report summarises recent (2015) results from the assessment of seagrass change at permanent transect sites established in 2009 as part of the Western Basin Dredging and Disposal Project (WBDDP). This work complements long‐term annual seagrass monitoring conducted since 2002 which examines and re‐maps the full extent of seagrasses each November at the time of their maximum distribution (see Davies et al. 2016). Seagrasses at permanent transects in Port Curtis and Rodds Bay showed distinct seasonal trends as well as significant inter‐annual changes in seagrass percent cover, above-ground biomass and species composition. Over the duration of the monitoring program (since November 2009), Gladstone has received higher than average rainfall during the majority of wet seasons, punctuated by two of the most extreme flood events on record. Significant declines in seagrass abundance occurred at all sites during the monitoring program,particularly following the 2010/2011 flood event. Some sites showed substantial recovery,particularly in the outer harbour following the 2010/2011 flood event however inner harbour sites have yet to recover to pre‐flood levels. The timing of flood related declines immediately prior to major WBDDP dredging activities makes it difficult to ascertain what additional impact dredging may have had on seagrass condition and recovery; however, in situ monitoring at permanent transect sites indicates that Zostera muelleri subsp. capricorni received enough light to meet their growth requirements during the dredging program. In January 2013, the Calliope River again discharged at record levels and declines in seagrass were detected across monitoring sites. Recovery over subsequent growing seasons has varied with some sites remaining atypically low in seagrass cover. Gladstone seagrasses were capable of the production of flowers, fruits and seeds. However this varied substantially between meadows and time of year. It is likely that propagule limitation at some sites may be inhibiting seagrass recovery. Sediment seed banks for Zostera muelleri subsp. capricorni were detected in the inner and outer harbour and at Rodds Bay at each quarterly monitoring event in 2015; however viability decreased across the senescent season (February to May 2015) especially in the outer harbour at Pelican Banks. Results of seagrass monitoring over the post‐dredging phase of the project will continue to provide insight into the capacity of seagrass resilience to human activities. If low levels of resilience detected at many sites persist then the tools and thresholds established through the Gladstone seagrass research programs will be critical in ensuring successful management of their recovery. Currently seagrasses have shown some capacity to recover from impacts in Gladstone, but as has been seen in other Queensland locations repeated disturbances over multiple years may lead to long‐term loss, with recovery trajectories far less certain. The extensive and detailed seagrass monitoring and research efforts in Gladstone means we are well placed to understand these processes and can look to implement measures to reduce the chances of exacerbating natural impacts by human activities.